1. Field of the Invention
The present invention relates generally to the field of laser oscillators and master-oscillator power amplifiers (MOPA's), and more particularly to laser oscillators and MOPA's using spatially inhomogeneous optical pumping beam patterns to minimize the potential for parasitic oscillation modes and amplified spontaneous emission (ASE) loss, leading to a new class of low-power, compact, high-efficiency oscillators and MOPA's, as well as to a new class of high-power, cw or Q-switched scalable laser systems.
2. Brief Description of the Prior Art
Typically, optical pump beams to be used in laser oscillators and master-oscillator power amplifiers are designed to be spatially uniform. This results in undesirable energy depletion in transverse (i.e., lateral) directions across the laser gain media both in oscillators as well as in MOPA's. This undesirable result is especially noticeable in systems with high-gain laser media, with large aspect ratios, which incorporate several disk amplification stages, and can make them inadequate for a given application. For example, optical sources for high-power welders, and other industrial and DoD laser applications, need to give output with high gain and/or great amount of power. However, the existing state-of-the-art optical systems used in these applications have high potential for parasitic modes and ASE loss, thus requiring design tradeoffs and use of less efficient optical sources.
Some conventional laser systems use dimension limiting schemes to avoid parasitics and ASE loss. The first method involves merely limiting the physical size of the gain medium, or the transverse spatial extent of the uniform pump beam. The article "Scalable Concept For Diode-Pumped High-Power Solid-State Lasers", by A. Giesen et al., published in Applied Physics B 58, 365-372, Springer-Verlag (1994), describes a three-level laser gain media element which employ thin disk stages attached to coolers. In this application, the size of the surface area of the disk has to be limited due to parasitics, while the thickness is limited by thermal considerations. These limitations in size dictate a reduction in size of the usable surface area of the gain medium, which results in a lower number of the usable pump photons. See also "Scalable High-Power Optically Pumped GaAs Laser" by Le, Di Cecca and Mooradian, published in Applied Physics Lett., Vol. 58, No. 18, 1967-1969, American Institute of Physics (1991).
Another method to circumvent undesirable transverse losses involves physically sectioning or otherwise modifying large-size gain medium into a number of smaller discrete gain cells, as described in U.S. Pat. No. 4,757,268 issued in 1988 to Abrams et al. As an example of the latter case, a large transverse area gain medium, such as Nd:YAG, is longitudinally sectioned or sliced into a number of small segments. In addition, loss elements (e.g., absorbing slabs) may be placed between the gain medium elements to avoid transverse parasitics of the package. Further, this technique also requires coherent combining of the discrete amplifying stages to realize optimal far-field performance, which is usually accomplished via adaptive optics or via nonlinear optical phase conjugation. The ensemble can then be coherently combined using a double-pass phase-conjugate MOPA configuration.
In yet another known conventional method of reducing ASE and parasitic oscillation modes, a large-area wafer with a MQW epilayer, which serves as the gain medium, is processed during growth to generate discrete gain regions which can yield gain under optical pumping, while other regions cannot, even in the presence of the pump beams. However, this procedure requires additional processing steps during epilayer growth, which adds cost and complexity to the system.